If you want to ride the gondola and upper tram, you'll need to buy a ticket to Fassa Bike Park.
4x branches off of Double U right at the top of the mountain. The trail has a similar character to Double U with tons of berms, rollers, and tabletops. The final section of trail features a little more of a natural feel, with rocks and roots, but it's short-lived.
When the trail spits you out onto the road at the end, you can either follow the road to Double U or hang a right and continue downhill on Infinity.